Ester compounds and their use in liquid-crystalline media

ABSTRACT

The invention relates to esters of the formula I and to a liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula I                    
     in which R, A 1 , Z 1 , L 1 , L 2 , Y and n are as defined in herein.

The present invention relates to ester compounds of the formula I andtheir use in a liquid-crystalline medium.

Liquid crystals are used, in particular, as dielectrics in displaydevices, since the optical properties of such substances can be modifiedby an applied voltage. Electro-optical devices based on liquid crystalsare extremely well known to the person skilled in the art and can bebased on various effects. Examples of such devices are cells havingdynamic scattering, DAP (deformation of aligned phases) cells,guest/host cells, TN cells having a twisted nematic structure, STN(supertwisted nematic) cells, SBE (superbirefringence effect) cells andOMI (optical mode interference) cells. The most common display devicesare based on the Schadt-Helfrich effect and have a twisted nematicstructure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and give short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the abovementioned cells, at conventionaloperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, must satisfy various requirements depending on thecell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, media having large positive dielectric anisotropy, broadnematic phases, relatively low bire-fringence, very high resistivity,good UV and temperature stability and low vapour pressure are desiredfor matrix liquid-crystal displays containing integrated non-linearelements for switching individual pixels (MLC displays).

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). This isthen referred to as an “active matrix”, and a distinction can be madebetween two types:

1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer assubstrate.

2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material limits thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,for example CdSe, or TFTs based on polycrystalline or amorphous silicon.Intensive work is being carried out worldwide on the latter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be expanded to fullycolour-compatible displays, in which a mosaic of red, green and bluefilters is arranged in such a way that each filter element is locatedopposite a switchable pixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are backlit.

The term MLC displays here covers any matrix display containingintegrated non-linear elements, i.e., besides the active matrix, alsodisplays containing passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (lap-tops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinadequate resistivity of the liquid-crystal mixtures [TOGASHI, S.,SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E.,WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff,Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of ThinFilm Transistors for Matrix Addressing of Television Liquid CrystalDisplays, p. 145 ff, Paris]. With decreasing resistance, the contrast ofan MLC display drops, and the problem of after-image elimination canoccur. Since the resistivity of the liquid-crystal mixture generallydrops over the life of an MLC display owing to interaction with theinterior surfaces of the display, a high (initial) resistance is veryimportant in order to obtain acceptable service lives. In particular inthe case of low-volt mixtures, it was hitherto impossible to achievevery high resistivity values. It is furthermore important that theresistivity increases as little as possible with increasing temperatureand after heating and/or UV exposure. The low-temperature properties ofthe mixtures of the prior art are also particularly disadvantageous. Itis required that crystallization and/or smectic phases do not occur,even at low temperatures, and that the temperature dependence of theviscosity is as low as possible. The MLC displays of the prior art thusdo not satisfy today's requirements.

Besides liquid-crystal displays which use backlighting, i.e. areoperated transmissively and optionally transflectively, there is alsoparticular interest in reflective liquid-crystal displays. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than backlitliquid-crystal displays of corresponding size and resolution. Since theTN effect is characterized by very good contrast, reflective displays ofthis type are readily legible even under bright ambient conditions. Thisis already known of simple reflective TN displays, as used, for example,in wristwatches and pocket calculators. However, the principle can alsobe applied to high-quality, higher-resolution active matrix addresseddisplays such as TFT displays. Here, as is already the case in theconventional transmissive TFT-TN displays, the use of liquid crystals oflow birefringence (Δn) is necessary in order to achieve low opticalretardation (d·Δn).

This low optical retardation results in a low viewing angle dependenceof the contrast, which is usually acceptable (cf. DE 30 22 818). Inreflective displays, the use of liquid crystals of low birefringence ismuch more important than in transmissive displays, since in reflectivedisplays, the effective layer thickness, through which the light passes,is approximately twice as large as in transmissive displays of the samelayer thickness.

Besides the lower power consumption (no need for backlight), otheradvantages of reflective displays over transmissive displays are thespace saving, which results in a very low installation depth, and thereduction in problems arising from temperature gradients resulting fromdifferences in heating caused by the backlight.

There thus continues to be a great demand for MLC displays which havevery high resistivity at the same time as a broad operating temperaturerange, short response times, even at low temperatures, and a lowthreshold voltage, and which do not have these disadvantages, or only doso to a reduced extent.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

expanded nematic phase range (in particular down to low temperatures)

switchability at extremely low temperatures (outdoor use, automobile,avionics)

increased resistance to UV radiation (longer life)

lower threshold (addressing) voltage

lower birefringence so as to improve the observation angle range.

The media available from the prior art do not allow these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired which enablegreater multiplexability and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther extension of the parameter latitude available (clearing point,smectic-nematic transition or melting point, viscosity, dielectricparameters, elastic parameters) is urgently desired.

It is an object of the invention to provide media for these MLC, TN orSTN displays, in particular for reflective MLC displays, which do nothave the abovementioned disadvantages or only do so to a reduced extent,and preferably at the same time have very high resistivity values andlow threshold voltages.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

It has now been found that these objects can be achieved by using mediaaccording to the invention in displays. The mixtures of the inventionare particularly notable for their excellent low-temperature behaviour.

The invention thus includes a liquid-crystalline medium based on amixture of polar compounds having positive dielectric anisotropy,characterized in that it comprises one or more compounds of the generalformula I

in which

R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which isunsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted,upto pehalo-substituted, by halogen, where one or more CH₂ groups inthese radicals may also, in each case independently of one another, bereplaced by —O—, —S—,

—CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are notlinked directly to one another,

A¹

(a) is a trans-1,4-cyclohexylene radical in which, in addition, one ormore non-adjacent CH₂ groups may be replaced by —O— and/or —S—,

(b) is a 1,4-phenylene radical, in which, in addition, one or two CHgroups may be replaced by N,

(c) is a 1,4-cyclohexenylene radical,

(d) is a radical from the group consisting of1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl,decahydro-naphthalene-2,6-diyl and1,2,3,4-tetra-hydronaphthalene-2,6-diyl,

where the radicals (a), (b), (c) and (d) may be monosubstituted orpolysubstituted by CN or fluorine,

z¹ is —COO—, —O—CO—, —CH₂O—, —OCH₂—, —C₂H₄—, —CH═CH—, —CF₂O—, —OCF₂—,—C≡C—, —(CH₂)₄—, —CH═CHC₂H₄—, —C₂F₄— or a single bond,

L¹ or L² are each, independently of one another, H or F,

Y is F, Cl, CN or an alkyl or alkoxy radical having 1 to 6 carbon atomswhich is substituted by one or more halogen atoms, where one or more CH₂groups may also be replaced by —O— or —CH═CH— in such a way that O atomsare not linked directly to one another, and

n is 0, 1 or 2.

The compounds of the formula I, which the invention likewise provides,have a broad range of applications. Depending on the choice ofsubstituents, these compounds can serve as base materials of whichliquid-crystalline media are predominantly composed; however, it is alsopossible to add compounds of the formula I to liquid-crystalline basematerials from other classes of compounds in order, for example, tomodify the dielectric and/or optical anisotropy of a dielectric of thistype and/or to optimize its threshold voltage and/or its viscosity.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

Particular preference is given to compounds of the formula I in which Zis a single bond, furthermore —CH₂—CH₂—. n is preferably 0 or 1, A¹ ispreferably a cyclohexane ring or a dioxane ring.

Y is preferably F, Cl, CN, CF₃, CF₂H, OCF₃, OCF₂H, OCFHCF₃, OCFHCH₂F,OCFHCHF₂, OCF₂CH₃, OCF₂CH₂F, OCF₂CHF₂, OCF₂CF₂CF₂H, OCF₂CF₂CH₂F,OCFHCF₂CF₃, OCFHCF₂CHF₂, OCFHCFHCF₃, OCH₂CF₂CF₃, OCF₂CF₂CF₃,OCF₂CFHCHF₂, OCF₂CH₂CHF₂, OCFHCF₂CHF₂, OCFHCFHCHF₂, OCFHCH₂CF₃,OCH₂CFHCF₃, OCH₂CF₂CHF₂, OCF₂CFHCH₃, OCF₂CH₂CHF₂, OCFHCF₂CH₃,OCFHCFHCHF₂, OCFHCH₂CF₃, OCH₂CF₂CHF₂, OCH₂CFHCHF₂, OCF₂CH₂CH₃,OCFHCFHCH₃, OCFHCH₂CHF₂, OCH₂CF₂CH₃, OCH₂CFHCHF₂, OCH₂CH₂CHF₂,OCHCH₂CH₃, OCH₂CFHCH₃, OCH₂CH₂CHF₂, OCClFCF₃, OCClFCClF₂, OCClFCHF₂,OCFHCCl₂F, OCClFCHF₂, OCClFCClF₂, OCF₂CHCl₂, OCF₂CHCl₂, OCF₂CCl₂F,OCF₂CClFH, OCF₂CClF₂, OCF₂CF₂CCIF₂, OCF₂CF₂CCl₂F, OCClFCF₂CF₃,OCClFCF₂CHF₂, OCClFCF₂CClF₂, OCClFCFHCF₃, OCClFCClFCF₃, OCCl₂CF₂CF₃,OCClHCF₂CF₃, OCClFCF₂CF₃, OCClFCClFCF₃, OCF₂CClFCHF₂, OCF₂CF₂CCl₂F,OCF₂CCl₂CHF₂, OCF₂CH₂CClF₂, OCClFCF₂CFH₂, OCFHCF₂CCl₂F, OCClFCFHCHF₂,OCClFCClFCF₂H, OCFHCFHCClF₂, OCClFCH₂CF₃, OCFHCCl₂CF₃, OCCl₂CFHCF₃,OCH₂CClFCF₃, OCCl₂CF₂CF₂H, OCH₂CF₂CClF₂, OCF₂CClFCH₃, OCF₂CFHCCl₂H,OCF₂CCl₂CFH₂, OCF₂CH₂CCl₂F, OCClFCF₂CH₃, OCFHCF₂CCl₂H, OCClFCClFCHF₂,OCFHCFHCCl₂F, OCClFCH₂CF₃, OCFHCCl₂CF₃, OCCl₂CF₂CFH₂, OCH₂CF₂CCl₂F,OCCl₂CFHCF₂H, OCClHCClFCF₂H, OCF₂CClHCClH₂, OCF₂CH₂CCl₂H, OCClFCFHCH₃,OCF₂CClFCCl₂H, OCClFCH₂CFH₂, OCFHCCl₂CFH₂, OCCl₂CF₂CH₃, OCH₂CF₂CClH₂,OCCl₂CFHCFH₂, OCH₂CClFCFCl₂, OCH₂CH₂CF₂H, OCClHCClHCF₂H, OCH₂CCl₂CF₂H,OCClFCH₂CH₃, OCFHCH₂CCl₂H, OCClHCFHCClH₂, OCH₂CFHCCl₂H, OCCl₂CH₂CF₂H,OCH₂CCl₂CF₂H, CH═CF₂, OCH═CF₂, CF═CF₂, OCF═CF₂, CF═CHF, OCF═CHF, CH═CHF,OCH═CHF in particular F, Cl, CN, CF₃, CHF₂, OCF₃, OCHF₂, OCFHCF₃,OCFHCHF₂, OCFHCHF₂, OCF₂CH₃, OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂,OCF₂CF₂CHF₂, OCFHCF₂CF₃, OCFHCF₂CHF₂, OCF₂CF₂CF₃, OCF₂CF₂CClF₂,OCClFCF₂CF₃ or CH═CHF₂.

If R is an alkyl radical and/or an alkoxy radical, this can bestraight-chain or branched. It is preferably straight-chain, has 2, 3,4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl,butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxyor heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl,tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy,undecoxy, dodecoxy, tridecoxy or tetradecoxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4-or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-,3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, or2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R is an alkyl radical in which one CH₂ group has been replaced by—CH═CH—, this can be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 carbon atoms. Accordingly, it is inparticular vinyl, prop-1- or prop-2-enyl, but-1-, -2- or but-3-enyl,pent-1-, -2-, -3- or pent-4-enyl, hex-1-, -2-, -3-, -4- or hex-5-enyl,hept-1-, -2-, -3-, -4-, -5- or hept-6-enyl, oct-1-, -2-, -3-, -4-, -5-,-6- or oct-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or non-8-enyl,dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8-or dec-9-enyl.

If R is an alkyl radical in which one CH₂ group has been replaced by —O—and one has been replaced by —CO—, these are preferably adjacent. Thesethus contain an acyloxy group —CO—O— or an oxycarbonyl group —O—CO—.These are preferably straight-chain and have 2 to 6 carbon atoms. Theyare accordingly in particular acetyloxy, propionyloxy, butyryloxy,pentanoyloxy, hexanoyloxy, acetyloxymethyl, propionyloxymethyl,butyryloxymethyl, pentanoyloxymethyl, 2-acetyloxyethyl,2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetyloxy-propyl,3-propionyloxypropyl, 4-acetyloxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxy-carbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxy-carbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxy-carbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(meth-oxycarbonyl)butyl.

If R is an alkyl radical in which one CH₂ group has been replaced byunsubstituted or substituted —CH═CH— and an adjacent CH₂ group has beenreplaced by CO or CO—O or O—CO, this can be straight-chain or branched.It is preferably straight-chain and has 4 to 13 carbon atoms.Accordingly, they are in particular acryloyloxymethyl,2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl,5-acryloyloxypentyl, 6-acryloyloxy-hexyl, 7-acryloyloxyheptyl,8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl,methacryloyl-oxymethyl, 2-methacryloyloxyethyl,3-methacryloyl-oxypropyl, 4-methacryloyloxybutyl,5-methacryloyloxy-pentyl, 6-methacryloyloxyhexyl,7-methacryloyloxy-heptyl, 8-methacryloyloxyoctyl or9-methacryloyloxy-nonyl.

If R is an alkyl or alkenyl radical which is mono-substituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CNor CF₃ is in any position.

If R is an alkyl or alkenyl radical which is at least monosubstituted byhalogen, this radical is preferably straight-chain and halogen ispreferably F or Cl. In the case of polysubstitution, halogen ispreferably F. The resulting radicals also include perfluorinatedradicals. In the case of monosubstitution, the fluoro or chlorosubstituent can be in any desired position, but is preferably in theω-position.

Compounds of the formula I which contain pendant groups R which aresuitable for polymerization reactions are suitable for the preparationof the liquid-crystalline polymers.

Compounds of the formula I containing branched pendant groups R mayoccasionally be of importance owing to better solubility in theconventional liquid-crystalline base materials, but in particular aschiral dopants if they are optically active. Smectic compounds of thistype are suitable as components for ferro-electric materials.

Compounds of the formula I having S_(A) phases are suitable, forexample, for thermally addressed displays.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methyl-propoxy, 2-methylbutoxy,3-methylbutoxy, 2-methyl-pentoxy, 3-methylpentoxy, 2-ethylhexoxy,1-methylhexoxy or 1-methylheptoxy.

If R is an alkyl radical in which two or more CH₂ groups have beenreplaced by —O— and/or —CO—O—, this can be straight-chain or branched.It is preferably branched and has 3 to 12 carbon atoms. Accordingly, itis in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-bis-carboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxy-heptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis-(ethoxycarbonyl)ethyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis-(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxy-carbonyl)hexyl.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag,Stuttgart), to be precise under reaction conditions which are known andsuitable for said reactions. Use can also be made here of variants whichare known per se, but are not mentioned here in greater detail.

The compounds according to the present invention can be prepared, forexample, as follows:

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highresistivity which is located in the cell) which contain media of thistype, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention allow asignificant extension of the parameter latitude which is available.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and optical anisotropy andtreshold voltage are far superior to current prior art materials.

The requirement for a high clearing point, nematic phase at lowtemperature and simultaneously a low threshold voltage has hitherto onlybeen met inadequately. Although liquid-crystal mixtures such as MLC-6476and MLC-6625 (Merck KGaA, Darmstadt, Germany) have comparable clearingpoints and low-temperature stabilities, they have, however, both muchhigher Δn values of about 0.075 and much higher threshold voltages ofabout ≧1.7 V.

While maintaining the nematic phase down to −20° C., preferably down to−30° C., particularly preferably down to −40° C., the liquid-crystalmixtures according to the invention allow clearing points above 70° C.,preferably above 80° C., particularly preferably above 90° C.,simultaneously birefringence values of ≦0.100, preferably ≦0.095, veryparticularly preferably ≦0.091, and a low threshold voltage to beachieved, allowing excellent STN and MLC displays, in particularreflective MLC displays, to be achieved. In particular, the mixtures arecharacterized by low operating voltages. The TN thresholds are at about1.5 V, preferably below 1.3 V, particularly preferably <1.0 V.Reflective MLC mixtures are particularly notable for TN thresholds <1.5V.

It goes without saying that a suitable choice of the components of themixtures according to the invention also allows higher clearing points(for example above 110° C.) to be achieved at the same time as lowerdielectric anisotropy values and thus higher threshold voltages, orlower clearing points to be achieved at the same time as higherdielectric anisotropy values (for example >12) and thus lower thresholdvoltages (for example <1.5 V) while retaining the other advantageousproperties. Likewise, mixtures of higher Δε and thus lower thresholdscan be obtained at viscosities which are increased correspondinglylittle. The MLC displays according to the invention preferably operateat the first Gooch and Tarry transmission minimum [C. H. Gooch and H. A.Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl.Phys., Vol. 8, 1575-1584, 1975], where, besides particularly favourableelectro-optical properties, such as, for example, high steepness of thecharacteristic line and low angle dependence of the contrast (GermanPatent 30 22 818), a lower dielectric anisotropy is sufficient at thesame threshold voltage as in an analogous display at the second minimum.Thus, significantly higher resistivities can be achieved using themixtures according to the invention at the first minimum than in thecase of mixtures comprising cyano compounds. A person skilled in the artcan use simple routine methods to produce the birefringence necessaryfor a specified layer thickness of the MLC display by suitable choice ofthe individual components and their proportions by weight. Therequirements of reflective MLC displays are described, for example, inDigest of Technical Papers, SID Symposium 1998.

The rotational viscosity of the mixtures according to the invention at20° C. is preferably <200 mpa.s, particularly preferably <180 mPa.s. Thenematic phase range is preferably at least 90°, in particular at least100°. This range preferably extends at least from −20° to +80°.

Measurements of the capacity holding ratio, also known as the voltageholding ratio (HR) [S. Matsumoto et al., Liquid Crystals 5, 1320 (1989);K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304(1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)] have shown thatmixtures according to the invention comprising compounds of the formulaI have an HR which is sufficient for MLC displays.

The media according to the invention preferably contain a plurality(preferably two, three or more) of compounds of the formula I, i.e. theproportion of these compounds is 5-50%, preferably 5-40%, particularlypreferably in the range 5-35%.

The individual compounds of the formulae I to XV and their subformulaewhich can be used in the media according to the invention are eitherknown or can be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

Mixture comprising one or more compounds of the formulae Ia to Ik:

In the compounds of the formula I and of the sub-formulae Ia to Ik, R ispreferably a straight-chain alkyl radical having 1-8 carbon atoms or analkenyl radical having 2-8 carbon atoms. R is particularly preferablymethyl, ethyl, n-propyl, n-pentyl, vinyl, 1E-propenyl and 3-butenyl.

The medium comprises one or more compounds of the formula Ia, Ib, Icand/or Ik;

The medium additionally comprises one or more compounds selected fromthe group consisting of the general formulae II to VIII:

in which the individual radicals have the following meanings:

R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having upto 9 carbon atoms;

X⁰ is F, Cl, halogenated alkyl or alkoxy having 1 to 6 carbon atoms orhalogenated alkenyl having 2 to 6 carbon atoms;

Z⁰ is —C₄H₈—, CF₂O—, —OCF₂—, —C₂F₄— or —CH═CH—;

Y¹ to Y⁴ are each, independently of one another, H or F;

r is 0 or 1.

The medium preferably comprises two, three, four or five compounds ofthe formula II;

The medium preferably comprises one or more compounds of the formulaeIIa to IIh:

The compound of formula IV is preferably

The medium additionally comprises one or more compounds selected fromthe group consisting of the general formulae IX to XV:

in which R⁰, X⁰, Y¹ and y² are each, independently of one another, asdefined in claim 2. In the compounds of the formulae II to XV, X⁰ ispreferably F, Cl, CF₃, OCF₃, OCHF₂, in particular F and OCF₃,furthermore OCHF₂. R⁰ is preferably alkyl, oxaalkyl, fluoroalkyl oralkenyl, in each case having up to 6 carbon atoms.

The medium additionally comprises one or more compounds of the formula

in which R⁰ and X⁰ are as defined above.

The medium additionally comprises one or more ester compounds of theformulae E1 to E5:

in which R⁰, X⁰, y¹, y² and y³ are as defined above. Alkyl and alkyl*are each a straight-chain alkyl radical having 1 to 9 carbon atoms.

The medium additionally comprises one or more compounds of the formulaeXa to Xd:

The medium additionally comprises one or more compounds of the formulaE1a and/or E1b:

in which R⁰ and Y² are as defined above.

The proportion of compounds of the formulae I to VIII together in thetotal mixture is at least 50% by weight;

The proportion of compounds of the formula I in the total mixture isfrom 5 to 50% by weight;

The proportion of compounds of the formulae II to VIII in the totalmixture is from 20 to 80% by weight;

The medium comprises compounds of the formulae II, III, IV, V, VI, VIIor VIII;

R⁰ is preferably a straight-chain alkyl or alkenyl having 2 to 7 carbonatoms;

The medium essentially consists of compounds of the formulae I to VIII;

The medium comprises further compounds, preferably selected from thefollowing group consisting of the general formulae XVI to XIX:

in which R⁰ and X⁰ are as defined above and the 1,4-phenylene rings canbe substituted by CN, chlorine or fluorine. The 1,4-phenylene rings arepreferably monosubstituted or polysubstituted by fluorine atoms.

The weight ratio I: (II+III+IV+V+VI+VII +VIII) is preferably from 1:10to 10:1.

The medium essentially consists of compounds selected from the groupconsisting of the general formulae I to XV.

The proportion of compounds of the formulae Xa to Xd in the totalmixture is 3-45% by weight, preferably 5-40% by weight, in particular5-30% by weight.

The proportion of compounds of the formula E1 in the total mixture is10-60% by weight, preferably 10-45% by weight, in particular 15-40% byweight.

The proportion of compounds of the formulae E2 and/or E3 in the totalmixture is 1-30% by weight, preferably 3-20% by weight, in particular3-15% by weight.

The proportion of compounds of the formula E4 is preferably ≦20% byweight, in particular ≦10% by weight.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formula II, III, IV, V,VI, VII and/or VIII, leads to a decrease in the threshold voltage and tolow birefringence values, where broad nematic phases with lowsmectic-nematic transition temperatures are simultaneously observed,which drastically improves the storage stability. Particularly preferredare mixtures which, in addition to one or more compounds of the formulaI, comprise one or more compounds of the formula IV, in particularcompounds of the formula IVa, where X⁰ is F or OCF₃.

The compounds of the formulae I to VIII are colourless, stable andreadily miscible with one another and with other liquid-crystallinematerials.

The term “alkyl” or “alkyl*” encompasses straight-chain and branchedalkyl groups having 1-7 carbon atoms, particularly the straight-chaingroups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groupshaving 2-5 carbon atoms are generally preferred.

The term “alkenyl” or “alkenyl*” encompasses straight-chain and branchedalkenyl groups having 2-7 carbon atoms, in particular the straight-chaingroups. Particularly preferred alkenyl groups are C₂-C₇-1E-alkenyl,C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇6-alkenyl, inparticular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term “fluoroalkyl” preferably encompasses straight-chain groups withterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoro-pentyl, 6-fluorohexyl and 7-fluoroheptyl. Otherpositions of fluorine are not precluded, however.

The term “oxaalkyl” preferably encompasses straight-chain radicals ofthe formula C_(n)H_(2n+1)—O—(CH₂)_(m), where n and m are each,independently of one another, from 1 to 6. Preferably, n=1 and m is 1 to6.

A suitable choice of the meanings of R⁰ and X₀ allows the responsetimes, the threshold voltage, the slope of the transmissioncharacteristic lines etc. to be modified as desired. For example,1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and thelike generally result in shorter response times, improved nematictendencies and a higher ratio of the elastic constants k₃₃ (bend) andk₁₁ (splay) compared with alkyl or alkoxy radicals. 4-alkenyl radicals,3-alkenyl radicals and the like generally result in lower thresholdvoltages and smaller values of k₃₃/k₁₁ compared with alkyl and alkoxyradicals.

The optimum weight ratio of compounds of the formulae I andII+III+IV+V+VI+VII+VIII largely depends on the desired properties, onthe choice of the components of the formulae I, II, III, IV, V, VI, VIIand/or VIII, and on the choice of any other components which may bepresent. Suitable weight ratios within the range given above can easilybe determined from case to case.

The total amount of compounds of the formulae I to XV in the mixturesaccording to the invention is not critical. The mixtures can thereforecomprise one or more further components in order to optimize variousproperties. However, the observed effect on the response times and thethreshold voltage is usually greater the higher the total concentrationof compounds of the formulae I to XV.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VIII (preferably II,III and/or IV, especially IVa) in which X⁰ is F, OCF₃, OCHF₂, F,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic effect with thecompounds of the formula I results in particularly advantageousproperties. Mixtures comprising compounds of the formula I and of theformula IVa are particularly notable for their low threshold voltages.

The construction of the STN or MLC display according to the inventionfrom polarizers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term conventional construction is broadly drawn here and also coversall variations and modifications of the MLC display, in particularincluding matrix display elements based on poly-Si TFT or MIM andespecially reflective displays.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in a lesser amount is dissolved inthe components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again after thorough mixing, forexample by distillation. It is also possible to prepare the mixtures inother conventional manners, for example by using pre-mixtures, forexample homologue mixtures, or using so-called “multi-bottle” systems.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature. For example, 0-15%,preferably 0-10%, of pleochroic dyes and/or chiral dopants can be added.The additives are each employed in concentrations of from 0.01 to 6%,preferably from 0.1 to 3%. However, the concentration data for the otherconstituents of the liquid-crystal mixtures, i.e. of theliquid-crystalline or mesogenic compounds, are given without taking intoaccount the concentration of these additives.

C denotes a crystalline phase, S a smectic phase, S_(c) a smectic Cphase, N a nematic phase and I the isotropic phase.

The entire disclosure of all applications, patents and publications,cited above, and of corresponding German application No. DE 19954906.0,filed Nov. 16, 1999, is hereby incorporated by reference.

In the present application and in the following examples, the structuresof the liquid crystal compounds are specified by acronyms, which can betransformed into chemical formulae according to the following Tables Aand B. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) are straight-chainalkyl radicals having n or m C atoms. n and m are integers, preferably0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, where n=m or n≠m. The codingaccording to Table B is self-evident. Table A specifies the acronym forthe parent body only. In individual cases, the acronym for the parentbody is followed, separated therefrom by a hyphen, by a code for thesubstituents R¹, R², L¹ and L²:

Code for R¹, R², L¹, L² R¹ R² L¹ L² nm C_(n)H_(2n+1) C_(m)H_(2m+1) H HnOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) HH n C_(n)H_(2n+1) CN H H nN.F C_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F HH nOF OC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) FH F nF.F.F C_(n)H_(2n+1) F F F nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1)NCS H H rVsN C_(r)H_(2r+1)—CH═CH—C_(s)H_(2s)— CN H H V-T CH₂═CH CF₃ H HV2-T CH₂—CH—C₂H₄ CF₃ H H 1V-OT CH₃—CH═CH OCF₃ H H rEsNC_(r)H_(2r+1)—O—C₂H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) H HnOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

Preferred mixture components are shown in Tables A and B.

TABLE A

TABLE B

TABLE C Table C lists possible dopants which are usually added to themixtures according to the invention.

Particular preference is given to mixtures according to the inventionwhich, in addition to one or more compounds of the formula I, comprisetwo, three or more compounds selected from Table B.

The following examples are intended to illustrate the invention withoutlimiting it. Hereinbefore and hereinafter, percentages are given in percent by weight. All temperatures are specified in degrees Celsius. m.p.denotes melting point, cl.p.=clearing point. Furthermore, C=crystallinestate, N=nematic phase, S=smectic phase and I=isotropic phase. The databetween these symbols represent the transition temperatures. The opticalanisotropy (589 nm, 20° C.), and the flow viscosity ν₂₀ (mm²/sec) andthe rotational viscosity γ₁ (mPa·s) were each determined at 20° C. ν₁₀denotes the voltage for 10% transmission (viewing directionperpendicular to the substrate surface). t_(on) denotes the on time andt_(off) the off time at an operating voltage corresponding to twice thevalue of V₁₀. Δn denotes the optical anisotropy and n₀ the refractiveindex. Δε denotes the dielectric anisotropy (Δε=ε_(II)—ε_, where ε_(II)refers to the dielectric constant parallel to the longitudinal axes ofthe molecule and ε₁ is the dielectric constant perpendicular thereto).The electro-optical data were measured in a TN cell in the 1st minimum(i.e. at a d·Δn value of 0.5) at 20° C., unless expressly statedotherwise. The optical data were measured at 20° C., unless expresslystated otherwise.

EXAMPLES Example 1

Step 1.1

90 ml of concentrated sulfuric acid are added dropwise to an initialcharge of 62.2 ml of nitric acid at about 0° C. (nitrating acid). Thenitrating acid is added dropwise at about 0° C. to a suspension of 1 molof A in 1.1 l of dichloromethane. The mixture is stirred for 2 h atabout 0° C. and then admixed with ice-water. After extracting withdichloromethane and then with water, the crystals are recrystallizedfrom toluene.

Step 1.2

0.5 mol of B are hydrogenated in 1.3 l of abs. THF in the presence of20.7 g Pd-C (5%) catalyst.

After hydrogenation is complete, the catalyst is filtered off and thesolvent is removed under reduced pressure. The product is subjected tocustomary work-up.

Step 1.3

An initial charge of 250 ml of hydrogen fluoride (65% strength solutionin pyridine) is cooled to 5° C. After addition of 0.175 mol of C,stirring is continued for another 1.5 h. At 10° C., 0.175 mol sodiumnitrite is added portionwise to the reaction solution at 5-10° C. Thereaction solution is then slowly warmed to 60° C., stirred at thistemperature for 0.5 h and then stirred at room temperature overnight.After addition of ice, the mixture is subjected to customary work-up.The product is recrystallized from ethyl acetate.

Step 1.4

A solution of 0.031 mol of N,N′-dicyclohexylcarbo-diimide in 15 ml ofdichloromethane is added dropwise at 10° C. to a suspension of 0.025 molof D, 0.026 mol of 3,4,5-trifluorophenol and 0.001 mol of4-(dimethyl-amino) -pyridine in 55 ml of dichloromethane, produced atroom temperature.

The reaction solution is stirred at room temperature overnight, admixedwith 0.026 mol of oxalic acid dihydrate and stirred for another 10minutes. Finally, the mixture is subjected to customary work-up. Theproduct is recrystallized from n-hexane. C 85 N (67.2) I; Δε=15.18;Δn=0.13.

The following compounds of the formula

are prepared in a similar manner:

R n Y L¹ L² CH₃ 1 F H H CH₃ 1 F H F CH₃ 1 F F F C₂H₅ 1 F H H C₂H₅ 1 F HF C₂H₅ 1 F F F C 98.1; Δε = 14.67; Δn = 0.12

R n Y L¹ L² n-C₃H₇ 1 F H H n-C₃H₇ 1 F H F n-C₄H₉ 1 F H H n-C₄H₉ 1 F H Fn-C₄H₉ 1 F F F n-C₅H₁₁ 1 F H H n-C₅H₁₁ 1 F H F C58 N 106, I; Δε = 8.92;Δn = 0.12 n-C₅H₁₁ 1 F F F C71 N 81.6; I; Δε = 14.61; Δn = 0.12 n-C₆H₁₃ 1F H H n-C₆H₁₃ 1 F H F n-C₆H₁₃ 1 F F F CH₂═CH 1 F H H CH₂═CH 1 F H FCH₂═CH 1 F F F CH₃CH═CH 1 F H H CH₃CH═CH 1 F H F CH₃CH═CH 1 F F FCH₂═CHC₂H₄ 1 F H H CH₂═CHC₂H₄ 1 F H F CH₂═CHC₂H₄ 1 F F F CH₃CH═CHC₂H₄ 1F H H CH₃CH═CHC₂H₄ 1 F H F CH₃CH═CHC₂H₄ 1 F F F CH₃O 1 F H H CH₃O 1 F HF CH₃O 1 F F F CH₃ 1 Cl H H CH₃ 1 Cl H F CH₃ 1 Cl F F C₂H₅ 1 Cl H H C₂H₅1 Cl H F C₂H₅ 1 Cl F F n-C₃H₇ 1 Cl H H n-C₃H₇ 1 Cl H F n-C₃H₇ 1 Cl F Fn-C₄H₉ 1 Cl H H n-C₄H₉ 1 Cl H F n-C₄H₉ 1 Cl F F n-C₅H₁₁ 1 Cl H H n-C₅H₁₁1 Cl H F n-C₅H₁₁ 1 Cl F F n-C₆H₁₃ 1 Cl H H n-C₆H₁₃ 1 Cl H F n-C₆H₁₃ 1 ClF F CH₂═CH 1 Cl H H CH₂═CH 1 Cl H F CH₂═CH 1 Cl F F CH₃CH═CH 1 Cl H HCH₃CH═CH 1 Cl H F CH₃CH═CH 1 Cl F F CH₂═CHC₂H₄ 1 Cl H H CH₂═CHC₂H₄ 1 ClH F CH₂═CHC₂H₄ 1 Cl F F CH₃CH═CHC₂H₄ 1 Cl H H CH₃CH═CHC₂H₄ 1 Cl H FCH₃CH═CHC₂H₄ 1 Cl F F CH₃O 1 Cl H H CH₃O 1 Cl H F CH₃O 1 Cl F F CH₃ 1OCF₃ H H CH₃ 1 OCF₃ H F CH₃ 1 OCF₃ F F C₂H₅ 1 OCF₃ H H C58 S_(A) 88 N115 I; Δε = 8.57; Δn = 0.13 C₂H₅ 1 OCF₃ H F C₂H₅ 1 OCF₃ F F n-C₃H₇ 1OCF₃ H H C75 S_(A) 80 N 143 I; Δε = 8.66; Δn = 0.13 n-C₃H₇ 1 OCF₃ H Fn-C₃H₇ 1 OCF₃ F F n-C₄H₉ 1 OCF₃ H H n-C₄H₉ 1 OCF₃ H F n-C₄H₉ 1 OCF₃ F Fn-C₅H₁₁ 1 OCF₃ H H C84 S_(A) 86 N 140.8 I; Δε = 7.91; Δn = 0.12 n-C₅H₁₁1 OCF₃ H F n-C₅H₁₁ 1 OCF₃ F F n-C₆H₁₃ 1 OCF₃ H H n-C₆H₁₃ 1 OCF₃ H Fn-C₆H₁₃ 1 OCF₃ F F CH₂═CH 1 OCF₃ H H CH₂═CH 1 OCF₃ H F CH₂═CH 1 OCF₃ F FCH₃CH═CH 1 OCF₃ H H CH₃CH═CH 1 OCF₃ H F CH₃CH═CH 1 OCF₃ F F CH₂═CHC₂H₄ 1OCF₃ H H CH₂═CHC₂H₄ 1 OCF₃ H F CH₂═CHC₂H₄ 1 OCF₃ F F CH₃CH═CHC₂H₄ 1 OCF₃H H CH₃CH═CHC₂H₄ 1 OCF₃ H F CH₃CH═CHC₂H₄ 1 OCF₃ F F CH₃O 1 OCF₃ H H CH₃O1 OCF₃ H F CH₃O 1 OCF₃ F F CH₃ 1 OCHF₂ H H CH₃ 1 OCHF₂ H F CH₃ 1 OCHF₂ FF C₂H₅ 1 OCHF₂ H H C₂H₅ 1 OCHF₂ H F C₂H₅ 1 OCHF₂ F F n-C₃H₇ 1 OCHF₂ H Hn-C₃H₇ 1 OCHF₂ H F n-C₃H₇ 1 OCHF₂ F F n-C₄H₉ 1 OCHF₂ H H n-C₄H₉ 1 OCHF₂H F n-C₄H₉ 1 OCHF₂ F F n-C₅H₁₁ 1 OCHF₂ H H n-C₅H₁₁ 1 OCHF₂ H F n-C₅H₁₁ 1OCHF₂ F F n-C₆H₁₃ 1 OCHF₂ H H n-C₆H₁₃ 1 OCHF₂ H F n-C₆H₁₃ 1 OCHF₂ F FCH₂═CH 1 OCHF₂ H H CH₂═CH 1 OCHF₂ H F CH₂═CH 1 OCHF₂ F F CH₃CH═CH 1OCHF₂ H H CH₃CH═CH 1 OCHF₂ H F CH₃CH═CH 1 OCHF₂ F F CH₂═CHC₂H₄ 1 OCHF₂ HH CH₂═CHC₂H₄ 1 OCHF₂ H F CH₂═CHC₂H₄ 1 OCHF₂ F F CH₃CH═CHC₂H₄ 1 OCHF₂ H HCH₃CH═CHC₂H₄ 1 OCHF₂ H F CH₃CH═CHC₂H₄ 1 OCHF₂ F F CH₃O 1 OCHF₂ H H CH₃O1 OCHF₂ H F CH₃O 1 OCHF₂ F F CH₃ 1 CN H H CH₃ 1 CN H F CH₃ 1 CN F F C₂H₅1 CN H H C₂H₅ 1 CN H F C₂H₅ 1 CN F F n-C₃H₇ 1 CN H H n-C₃H₇ 1 CN H Fn-C₃H₇ 1 CN F F n-C₄H₉ 1 CN H H n-C₄H₉ 1 CN H F n-C₄H₉ 1 CN F F n-C₅H₁₁1 CN H H n-C₅H₁₁ 1 CN H F n-C₅H₁₁ 1 CN F F n-C₆H₁₃ 1 CN H H n-C₆H₁₃ 1 CNH F n-C₆H₁₃ 1 CN F F CH₂═CH 1 CN H H CH₂═CH 1 CN H F CH₂═CH 1 CN F FCH₃CH═CH 1 CN H H CH₃CH═CH 1 CN H F CH₃CH═CH 1 CN F F CH₂═CHC₂H₄ 1 CN HH CH₂═CHC₂H₄ 1 CN H F CH₂═CHC₂H₄ 1 CN F F CH₃CH═CHC₂H₄ 1 CN H HCH₃CH═CHC₂H₄ 1 CN H F CH₃CH═CHC₂H₄ 1 CN F F CH₃O 1 CN H H CH₃O 1 CN H FCH₃O 1 CN F F CH₃ 0 F H H CH₃ 0 F H F CH₃ 0 F F F C₂H₅ 0 F H H C₂H₅ 0 FH F C₂H₅ 0 F F F n-C₃H₇ 0 F H H n-C₃H₇ 0 F H F n-C₃H₇ 0 F F F n-C₄H₉ 0 FH H n-C₄H₉ 0 F H F n-C₄H₉ 0 F F F n-C₅H₁₁ 0 F H H n-C₅H₁₁ 0 F H Fn-C₅H₁₁ 0 F F F Δε = 31.0; Δn = 0.165 n-C₆H₁₃ 0 F H H n-C₆H₁₃ 0 F H Fn-C₆H₁₃ 0 F F F CH₂═CH 0 F H H CH₂═CH 0 F H F CH₂═CH 0 F F F CH₃CH═CH 0F H H CH₃CH═CH 0 F H F CH₃CH═CH 0 F F F CH₂═CHC₂H₄ 0 F H H CH₂═CHC₂H₄ 0F H F CH₂═CHC₂H₄ 0 F F F CH₃CH═CHC₂H₄ 0 F H H CH₃CH═CHC₂H₄ 0 F H FCH₃CH═CHC₂H₄ 0 F F F CH₃O 0 F H H CH₃O 0 F H F CH₃O 0 F F F CH₃ 0 Cl H HCH₃ 0 Cl H F CH₃ 0 Cl F F C₂H₅ 0 Cl H H C₂H₅ 0 Cl H F C₂H₅ 0 Cl F Fn-C₃H₇ 0 Cl H H n-C₃H₇ 0 Cl H F n-C₃H₇ 0 Cl F F n-C₄H₉ 0 Cl H H n-C₄H₉ 0Cl H F n-C₄H₉ 0 Cl F F n-C₅H₁₁ 0 Cl H H n-C₅H₁₁ 0 Cl H F n-C₅H₁₁ 0 Cl FF n-C₆H₁₃ 0 Cl H H n-C₆H₁₃ 0 Cl H F n-C₆H₁₃ 0 Cl F F CH₂═CH 0 Cl H HCH₂═CH 0 Cl H F CH₂═CH 0 Cl F F CH₃CH═CH 0 Cl H H CH₃CH═CH 0 Cl H FCH₃CH═CH 0 Cl F F CH₂═CHC₂H₄ 0 Cl H H CH₂═CHC₂H₄ 0 C H F CH₂═CHC₂H₄ 0 ClF F CH₃CH═CHC₂H₄ 0 Cl H H CH₃CH═CHC₂H₄ 0 Cl H F CH₃CH═CHC₂H₄ 0 Cl F FCH₃O 0 Cl H H CH₃O 0 Cl H F CH₃O 0 Cl F F CH₃ 0 OCF₃ H H CH₃ 0 OCF₃ H FCH₃ 0 OCF₃ F F C₂H₅ 0 OCF₃ H H C₂H₅ 0 OCF₃ H F C₂H₅ 0 OCF₃ F F n-C₃H₇ 0OCF₃ H H n-C₃H₇ 0 OCF₃ H F n-C₃H₇ 0 OCF₃ F F n-C₄H₉ 0 OCF₃ H H n-C₄H₉ 0OCF₃ H F n-C₄H₉ 0 OCF₃ F F n-C₅H₁₁ 0 OCF₃ H H Δε = 20.0; Δn = 0.171n-C₅H₁₁ 0 OCF₃ H F n-C₅H₁₁ 0 OCF₃ F F n-C₆H₁₃ 0 OCF₃ H H n-C₆H₁₃ 0 OCF₃H F n-C₆H₁₃ 0 OCF₃ F F CH₂═CH 0 OCF₃ H H CH₂═CH 0 OCF₃ H F CH₂═CH 0 OCF₃F F CH₃CH═CH 0 OCF₃ H H CH₃CH═CH 0 OCF₃ H F CH₃CH═CH 0 OCF₃ F FCH₂═CHC₂H₄ 0 OCF₃ H H CH₂═CHC₂H₄ 0 OCF₃ H F CH₂═CHC₂H₄ 0 OCF₃ F FCH₃CH═CHC₂H₄ 0 OCF₃ H H CH₃CH═CHC₂H₄ 0 OCF₃ H F CH₃CH═CHC₂H₄ 0 OCF₃ F FCH₃O 0 OCF₃ H H CH₃O 0 OCF₃ H F CH₃O 0 OCF₃ F F CH₃ 0 OCHF₂ H H CH₃ 0OCHF₂ H F CH₃ 0 OCHF₂ F F C₂H₅ 0 OCHF₂ H H C₂H₅ 0 OCHF₂ H F C₂H₅ 0 OCHF₂F F n-C₃H₇ 0 OCHF₂ H H n-C₃H₇ 0 OCHF₂ H F n-C₃H₇ 0 OCHF₂ F F n-C₄H₉ 0OCHF₂ H H n-C₄H₉ 0 OCHF₂ H F n-C₄H₉ 0 OCHF₂ F F n-C₅H₁₁ 0 OCHF₂ H Hn-C₅H₁₁ 0 OCHF₂ H F n-C₅H₁₁ 0 OCHF₂ F F n-C₆H₁₃ 0 OCHF₂ H H n-C₆H₁₃ 0OCHF₂ H F n-C₆H₁₃ 0 OCHF₂ F F CH₂═CH 0 OCHF₂ H H CH₂═CH 0 OCHF₂ H FCH₂═CH 0 OCHF₂ F F CH₃CH═CH 0 OCHF₂ H H CH₃CH═CH 0 OCHF₂ H F CH₃CH═CH 0OCHF₂ F F CH₂═CHC₂H₄ 0 OCHF₂ H H CH₂═CHC₂H₄ 0 OCHF₂ H F CH₂═CHC₂H₄ 0OCHF₂ F F CH₃CH═CHC₂H₄ 0 OCHF₂ H H CH₃CH═CHC₂H₄ 0 OCHF₂ H F CH₃CH═CHC₂H₄0 OCHF₂ F F CH₃O 0 OCHF₂ H H CH₃O 0 OCHF₂ H F CH₃O 0 OCHF₂ F F CH₃ 0 CNH H CH₃ 0 CN H F CH₃ 0 CN F F C₂H₅ 0 CN H H C₂H₅ 0 CN H F C₂H₅ 0 CN F Fn-C₃H₇ 0 CN H H n-C₃H₇ 0 CN H F n-C₃H₇ 0 CN F F n-C₄H₉ 0 CN H H n-C₄H₉ 0CN H F n-C₄H₉ 0 CN F F n-C₅H₁₁ 0 CN H H n-C₅H₁₁ 0 CN H F n-C₅H₁₁ 0 CN FF n-C₆H₁₃ 0 CN H H n-C₆H₁₃ 0 CN H F n-C₆H₁₃ 0 CN F F CH₂═CH 0 CN H HCH₂═CH 0 CN H F CH₂═CH 0 CN F F CH₃CH═CH 0 CN H H CH₃CH═CH 0 CN H FCH₃CH═CH 0 CN F F CH₂═CHC₂H₄ 0 CN H H CH₂═CHC₂H₄ 0 CN H F CH₂═CHC₂H₄ 0CN F F CH₃CH═CHC₂H₄ 0 CN H H CH₃CH═CHC₂H₄ 0 CN H F CH₃CH═CHC₂H₄ 0 CN F FCH₃O 0 CN H H CH₃O 0 CN H F CH₃O 0 CN F F

Mixture Examples Example A

CCH-35 5.0% Clearing point: +72.5° C. CCP-2F.F.F 10.0% Δn [589 nm, 20°C.]: +0.0902 CCP-3F.F.F 9.0% Δε [1 kHz, 20° C.]: +12.7 CCP-3OCF₃ 8.0% γ₁[mPas · s, 20° C.]: 167 CCP-4OCF₃ 4.0% d · Δn [μm, 20° C.]: 0.50 CGU-2-F11.0% Twist [°]: 90 CGU-3-F 9.0% V_(10,0,20) [V]: 1.02 CCZU-2-F 5.0% V₀[V]: 0.85 CCZU-3-F 15.0% CCZU-5-F 4.0% CGIZU-3-F 20.0%

Example B

PCH-5F 3.19% Δε [1 kHz, 20° C.]: +10.5 CCP-20CF₂.F.F 17.01% V₀ [V]: 1.11CCP-30CF₂.F.F 15.97% CCP-50CF₂.F.F 17.01% CUP-2F.F 5.35% CUP-3F.F 5.35%CBC-33F 5.35% CBC-53F 5.35% CBC-55F 5.27% CGIZU-3-F 20.15%

Example C

BCH-3F.F 10.82% γ₁ [mPa · s, 20° C.]: 142 BCH-5F.F 9.02% ECCP-30CF₃4.51% ECCP-50CF₃ 4.51% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% PCH-6F7.21% PCH-7F 5.41% CCP-20CF₃ 7.21% CCP-30CF₃ 10.82% CCP-40CF₃ 6.31%CCP-50CF₃ 9.92% PCH-5F 9.02% CGIZU-3-F 9.83%

Example D

BCH-3F.F 10.77% Δn [589 nm, 20° C.]: +0.0989 BCH-5F.F 8.98% Δε [1 kHz,20° C.]: +6.2 ECCP-30CF₃ 4.49% ECCP-50CF₃ 4.49% CBC-33F 1.80% CBC-53F1.80% CBC-55F 1.80% PCH-6F 7.18% PCH-7F 5.39% CCP-20CF₃ 7.18% CCP-30CF₃10.77% CCP-40CF₃ 6.28% CCP-50CF₃ 9.87% PCH-5F 8.98% CGIZU-3-F 10.23%

What is claimed is:
 1. A liquid-crystalline ester compound of theformula I

in which R is an alkyl or alkenyl radical having 1 to 15 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or monosubstitutedto perhalosubstituted by halogen, where one or more CH₂ groups in theseradicals, in each case independently of one another, are optionallyreplaced by —O—, —S—,

—CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are notlinked directly to one another, A¹ (a) is a trans-1,4-cyclohexyleneradical in which, one or more non-adjacent CH₂ groups are optionallyreplaced by —O— and/or —S—, (b) is a 1,4-phenylene radical, in which, inaddition, one or two CH groups may be replaced by N, (c) is a1,4-cyclohexenylene radical, (d) is a radical selected from the groupconsisting of 1,4-bicyclo[2.2.2]-octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and1,2,3,4-tetrahydronaphthalene-2,6-diyl, where the radicals (a), (b), (c)and (d) are optionally monosubstituted or polysubstituted by CN orfluorine, Z¹ is —COO—, —O—CO—, —CH₂O—, —OCH₂—, —C₂H₄—, —CH═CH—, —CF₂O—,—OCF₂—, —C≡C—, —(CH₂)₄—, —CH═CHC₂H₄—, —C₂F₄— or a single bond, L¹ or L²are each, independently of one another, H or F, Y is F, Cl, CN or analkyl or alkoxy radical having 1 to 6 carbon atoms which is substitutedby one or more halogen atoms, where one or more CH₂ groups areoptionally replaced by —O— or —CH═CH— in such a way that O atoms are notlinked directly to one another, and n is 0, 1 or 2, provided that, whenY═CN, L¹ and/or L² is F.
 2. An ester compound according to claim 1,wherein A¹ is a trans-1,4-cyclohexylene radical, n=1 and Z¹=single bond.3. An ester compound according to claim 1 wherein L¹ and/or L² are F. 4.An ester compound according to claim 1, wherein Z¹ is a single bond. 5.An ester compound according to claim 1, wherein Y is F, CN, OCF₃ orOCHF₂.
 6. A liquid-crystalline medium comprising at least one estercompound of claim
 1. 7. A liquid-crystalline medium based on a mixtureof polar compounds having positive dielectric anisotropy, whichcomprises one or more compounds of the formula I of claim 1 andadditionally one or more compounds selected from the group consisting ofcompounds of the formulae II, III, IV, V, VI, VII and VIII:

in which the individual radicals have the following meanings: R⁰ isn-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having 1 to 9carbon atoms, X⁰ is F, Cl, or halogenated alkyl, alkenyl or alkoxyhaving 1 to 6 carbon atoms, Z⁰ is —C₄H₈—, —CF₂O—, —OCF₂—, —C₂F₄— or—CH═CH—, Y¹ to Y⁴ are each, independently of one another, H or F, r is 0or
 1. 8. A liquid-crystalline medium according to claim 7, wherein theproportion of compounds of the formulae I to VIII together in the totalmixture is at least 50% by weight.
 9. A liquid-crystalline mediumaccording to claim 7, wherein the proportion of compounds of the formulaI in the total mixture is from 5 to 50% by weight.
 10. Aliquid-crystalline medium according to claim 7, wherein the proportionof compounds of the formulae II to VIII in the total mixture is from 20to 80% by weight.
 11. A liquid-crystalline medium according to claim 7,which additionally comprises one or more compounds of the formula E1

in which R⁰, X⁰ and y² are as defined in claim
 7. 12. Aliquid-crystalline medium according to claim 11, wherein for formula E1X⁰ is F or OCF₃ and Y² is H or F.
 13. A liquid-crystalline mediumaccording to claim 7, which additionally comprises one or more compoundsof the formula IVa

in which R⁰ and X⁰ are as defined in claim
 7. 14. A liquid-crystallinemedium according to claim 7, wherein the compound of the formula I isselected from the group consisting of compounds of formulas Ia to Ik:

where R is as defined.
 15. An electro-optical device comprising aliquid-crystalline medium according to claim
 7. 16. An electro-opticalliquid-crystal display containing a liquid-crystalline medium accordingto claim
 7. 17. A liquid-crystalline medium according to claim 7 whichmaintains a nematic phase down to −20° C., has a clearing point above70° C., a birefringence of ≦0.1 and a threshold voltage of about 1.5V orless.
 18. An ester compound of claim 1, wherein n is 1 or 2 and Z¹ is—CH₂—CH₂— or a single bond.
 19. A liquid crystalline medium of claim 7,wherein the medium comprises at least one compound of the formulae II,III and/or IV in which X⁰ is F, OCF₃, OCHF₂, F, OCH═CF₂, OCF═CF₂ orOCF₂—CF₂H.
 20. A liquid crystalline medium of claim 7, wherein themedium consists essentially of compounds of the formula I and compoundsof the formulae II, III, IV, V, VI, VII and VIII.